Trask phosphorylation defines the reverse mode of a phosphotyrosine signaling switch that underlies cell anchorage state <link href="file

Phosphotyrosine signaling in anchored epithelial cells constitutes a spacially ordained signaling program that largely functions to promote integrin-linked focal adhesion complexes, serving to secure cell anchorage to matrix and as a bidirectional signaling hub that coordinates the physical state of the cell and its environment with cellular functions including proliferation and survival. Cells release their adhesions during processes such as mitosis, migration, or tumorigenesis, but the fate of signaling through tyrosine phosphorylation in unanchored cells remains poorly understood. In an examination of epithelial cells in the unanchored state, we find abundant phosphotyrosine signaling, largely recommitted to an anti-adhesive function mediated through the Src family phosphorylation of their transmembrane substrate Trask/CDCP1/gp140. Src-Trask phosphorylation inhibits integrin clustering and focal adhesion assembly and signaling, defining an active phosphotyrosine signaling program underlying the unanchored state. Src-Trask signaling and Src-focal adhesion signaling inactivate each other, constituting two opposing modes of phosphotyrosine signaling that define a switch underline cell anchorage state. Src kinases are prominent drivers of both signaling modes, identifying their position at the helm of adhesion signaling capable of specifying anchorage state through substrate selection. These experimental studies along with concurring phylogenetic evidence suggest that phosphorylation on tyrosine is a signaling function fundamentally linked with the regulation of integrins.     

Fat facets and Liquid facets promote Delta endocytosis and Delta signaling in the signaling cells

Endocytosis modulates the Notch signaling pathway in both the signaling and receiving cells. One recent hypothesis is that endocytosis of the ligand Delta by the signaling cells is essential for Notch activation in the receiving cells. Here, we present evidence in strong support of this model. We show that in the developing Drosophila eye Fat facets (Faf), a deubiquitinating enzyme, and its substrate Liquid facets (Lqf), an endocytic epsin, promote Delta internalization and Delta signaling in the signaling cells. We demonstrate that while Lqf is necessary for three different Notch/Delta signaling events at the morphogenetic furrow, Faf is essential only for one: Delta signaling by photoreceptor precluster cells, which prevents recruitment of ectopic neurons. In addition, we show that the ubiquitin-ligase Neuralized (Neur), which ubiquitinates Delta, functions in the signaling cells with Faf and Lqf. The results presented bolster one model for Neur function in which Neur enhances Delta signaling by stimulating Delta internalization in the signaling cells. We propose that Faf plays a role similar to that of Neur in the Delta signaling cells. By deubiquitinating Lqf, which enhances the efficiency of Delta internalization, Faf stimulates Delta signaling.     

Cell-surface co-receptors: emerging roles in signaling and human disease

Extracellular signals are transmitted to cells through two classes of cell-surface receptors: signaling receptors that directly transduce signals and signaling co-receptors that bind ligand but that, traditionally, have not been thought to signal directly. Signaling co-receptors modulate the ligand binding and signaling of their respective signaling receptors. In recent years, roles for co-receptors have expanded to include essential functions in morphogen gradient formation, localizing signaling, signaling independently, regulating cell adhesion and orchestrating the signaling of several pathways. The importance of signaling co-receptors is demonstrated by their ubiquitous expression, their conservation during evolution, their prominent role in signaling cascades, their indispensable role during development and their frequent mutation or altered expression in human disease.     

Zebrafish peptidoglycan recognition protein SC (zfPGRP-SC) mediates multiple intracellular signaling pathways

Insect PGRPs can function as bacterial recognition molecules triggering proteolytic and/or signal transduction pathways, with the resultant production of antimicrobial peptides. To explore if zebrafish peptidoglycan recognition protein SC (zfPGRP-SC) has such effects, RNA interference (siRNA) and high-density oligonucleotide microarray analysis were used to identify differentially expressed genes regulated by zfPGRP-SC. The mRNA levels for a set of genes involved in Toll-like receptor signaling pathway, such as TLRs, SARM, MyD88, TRAF6 and nuclear factor (NF)-kappa B2 (p100/p52), were examined by quantitative RT-PCR (QT-PCR). The results from the arrays and QT-PCR showed that the expression of 133 genes was involved in signal transduction pathways, which included Toll-like receptor signaling, Wnt signaling, BMP signaling, insulin receptor signaling, TGF-beta signaling, GPCR signaling, small GTPase signaling, second-messenger-mediated signaling, MAPK signaling, JAK/STAT signaling, apoptosis and anti-apoptosis signaling and other signaling cascades. These signaling pathways may connect with each other to form a complex network to regulate not just immune responses but also other processes such as development and apoptosis. When transiently over-expressed in HEK293T cells, zfPGRP-SC inhibited NF-κB activity with and without lipopolysacharide (LPS) stimulation.     

Wnt/beta-catenin signaling interacts differentially with Ihh signaling in controlling endochondral bone and synovial joint formation

Both the Wnt/beta-catenin and Ihh signaling pathways play essential roles in crucial aspects of endochondral ossification: osteoblast differentiation, chondrocyte proliferation and hypertrophy. To understand the genetic interaction between these two signaling pathways, we have inactivated the beta-catenin gene and upregulated Ihh signaling simultaneously in the same cells during endochondral skeletal development using beta-catenin and patched 1 floxed alleles. We uncovered previously unexpected roles of Ihh signaling in synovial joint formation and the essential function of Wnt/beta-catenin signaling in regulating chondrocyte survival. More importantly, we found that Wnt and Ihh signaling interact with each other in distinct ways to control osteoblast differentiation, chondrocyte proliferation, hypertrophy, survival and synovial joint formation in the developing endochondral bone. Beta-catenin is required downstream of Ihh signaling and osterix expression for osteoblast differentiation. But in chondrocyte survival, beta-catenin is required upstream of Ihh signaling to inhibit chondrocyte apoptosis. In addition, Ihh signaling can inhibit chondrocyte hypertrophy and synovial joint formation independently of beta-catenin. However, there is a strong synergistic interaction between Wnt/beta-catenin and Ihh signaling in regulating synovial joint formation.     

Proteoglycan signaling co-receptors: Roles in cell adhesion, migration and invasion

Signaling co-receptors are diverse, multifunctional components of most major signaling pathways, with roles in mediating and regulating signaling in both physiological and pathophysiological circumstances. Many of these signaling co-receptors, including CD44, glypicans, neuropilins, syndecans and TßRIII/betaglycan are also proteoglycans. Like other co-receptors, these proteoglycan signaling co-receptors can bind multiple ligands, promoting the formation of receptor signaling complexes and regulating signaling at the cell surface. The proteoglycan signaling co-receptors can also function as structural molecules to regulate adhesion, cell migration, morphogenesis and differentiation. Through a balance of these signaling and structural roles, proteoglycan signaling co-receptors can have either tumor promoting or tumor suppressing functions. Defining the role and mechanism of action of these proteoglycan signaling co-receptors should enable more effective targeting of these co-receptors and their respective pathways for the treatment of human disease.     

Different levels of Notch signaling regulate quiescence, renewal and differentiation in pancreatic endocrine progenitors

Genetic studies have implicated Notch signaling in the maintenance of pancreatic progenitors. However, how Notch signaling regulates the quiescent, proliferative or differentiation behaviors of pancreatic progenitors at the single-cell level remains unclear. Here, using single-cell genetic analyses and a new transgenic system that allows dynamic assessment of Notch signaling, we address how discrete levels of Notch signaling regulate the behavior of endocrine progenitors in the zebrafish intrapancreatic duct. We find that these progenitors experience different levels of Notch signaling, which in turn regulate distinct cellular outcomes. High levels of Notch signaling induce quiescence, whereas lower levels promote progenitor amplification. The sustained downregulation of Notch signaling triggers a multistep process that includes cell cycle entry and progenitor amplification prior to endocrine differentiation. Importantly, progenitor amplification and differentiation can be uncoupled by modulating the duration and/or extent of Notch signaling downregulation, indicating that these processes are triggered by distinct levels of Notch signaling. These data show that different levels of Notch signaling drive distinct behaviors in a progenitor population.     

哺乳动物Hippo信号通路:肿瘤治疗的新标靶

Hippo信号通路是首次在果蝇中发现具有调节细胞增殖与凋亡作用的信号通路。最近发现果蝇Hippo信号通路的组成、分子作用机制和生物学功能在进化过程中高度保守。Hippo信号通路在胚胎发育中对细胞的生长分化、组织器官形成以及成体干细胞的维持和自稳态的保持等方面具有重要作用。同时,Hippo信号通路与Wnt信号通路、Notch信号通路等相互作用、密切联系,在肿瘤的发生、发展过程中也起到关键作用。文章综述了哺乳动物Hippo信号通路的作用机理、与其他信号通路和蛋白质因子的相互联系及与肿瘤的关系,对于肿瘤的诊断、预防和治疗具有一定的参考价值。     

Bone Morphogenetic Protein functions as a context-dependent angiogenic cue in vertebrates

Bone Morphogenetic Protein (BMP) signaling has been implicated in diverse biological processes. Although how BMP signaling regulates behaviors of endothelial cells during angiogenesis are not fully understood, increasing evidence indicate functions of BMP signaling components are essential in developmental and pathological angiogenesis. Here we review recent advances in delineating the functions of BMP signaling during angiogenesis. In addition, we discuss downstream pathways that transduce BMP signaling in endothelial cells, and factors that modulate BMP signaling response in endothelial cells. Finally, we provide recent insight on how BMP signaling functions as a context dependent angiogenic cue.     

Redox signaling

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) have recently been shown to be involved in a multiplicity of physiological responses through modulation of signaling pathways. Some of the specific signaling components altered by reactive oxygen and nitrogen species (RONS) have begun to be identified. We will discuss RONS signaling by detailing the chemistry of signaling, the roles of antioxidant enzymes as signaling components, thiol chemistry in the specificity of RONS signaling, .NO-heme interactions, and some do's and don'ts of redox signal research. The principal points raised are that: (1) as with classic signaling pathways, signaling by RONS is regulated; (2) antioxidant enzymes are essential 'turn-off components in signaling; (3) spatial relationships are probably more important in RONS signaling than the overall 'redox state' of the cell; (4) deprotonation of cysteines to form the thiolate, which can react with RONS, occurs in specific protein sites providing specificity in signaling; (5) although multiple chemical mechanisms exist for producing nitrosothiols, their formation in vivo remains unclear; and (6) caution should be taken in the use of 'antioxidants' in signal transduction.     

Hedgehog信号通路与肿瘤

Hedgehog信号通路在胚胎发育中细胞的生长分化、组织器官形成以及成体干细胞的维持和自稳态的保持等方面具有重要作用。同时,Hedgehog信号通路与Wnt信号通路、Notch信号通路等相互作用、密切联系,在肿瘤的发生、发展过程中也起到关键作用。论文综述了Hedgehog信号通路的作用机理,与其他信号通路、蛋白质因子的相互联系,以及在肿瘤研究中所关注的靶位点和小分子化合物抑制剂,对于癌症的预防和治疗具有一定的参考价值。     

Fluorescence imaging of signaling networks

Receptor-triggered signaling processes exhibit complex cross-talk and feedback interactions, with many signaling proteins and second messengers acting locally within the cell. The flow of information in this input-output system can only be understood by tracking where and when local signaling activities are induced. Systematic strategies are therefore needed to measure the localization and translocation of all signaling proteins, and to develop fluorescent biosensors that can track local signaling activities in individual cells. Such a biosensor tool chest can be based on two types of green fluorescent protein constructs that either translocate or undergo fluorescence-resonance-energy transfer when local signaling occurs. Broad strategies to measure quantitative, dynamic parameters in signaling networks, together with perturbation approaches, are needed to develop comprehensive models of signaling networks*.     

蜕皮激素和IIS-TORC1信号的分子互作

蜕皮激素信号主导调控昆虫的蜕皮和变态,决定昆虫的发育时间;IIS-TORC1信号整合生长因子、激素、营养和能量信号,决定昆虫的生长速率。蜕皮激素和IIS-TORC1信号之间发生3种分子互作:(1)IIS-TORC1信号促进前胸腺和卵巢合成蜕皮激素前体。(2)在蜕皮和变态期间,蜕皮激素抑制脂肪体细胞内IIS-TORC1信号、Myc的转录、细胞生长及其内分泌功能,导致脑神经分泌细胞分泌胰岛素样肽的功能减弱,从而降低昆虫全身性的IIS-TORC1信号。(3)在幼虫摄食期间,胰岛素信号抑制FOXO的转录活性,降低了蜕皮激素受体EcR的转录共激活因子DOR编码基因的转录水平,从而阻碍了蜕皮激素信号传导。蜕皮激素信号和IIS-TORC1信号协同调控发育时间和生长速率共同决定昆虫的个体大小。     

Signaling through sphingolipid microdomains of the plasma membrane: the concept of signaling platform

Transmembrane signaling requires modular interactions between signaling proteins, phosphorylation or dephosphorylation of the interacting protein partners and temporary elaboration of supramolecular structures, to convey the molecular information from the cell surface to the nucleus. Such signaling complexes at the plasma membrane are instrumental in translating the extracellular cues into intracellular signals for gene activation. In the most straightforward case, ligand binding promotes homodimerization of the transmembrane receptor which facilitates modular interactions between the receptor's cytoplasmic domains and intracellular signaling and adaptor proteins. For example, most growth factor receptors contain a cytoplasmic protein tyrosine kinase (PTK) domain and ligand-mediated receptor dimerization leads to cross phosphorylation of tyrosines in the receptor's cytoplasmic domains, an event that initiates the signaling cascade. In other signaling pathways where the receptors have no intrinsic kinase activity, intracellular nonreceptor PTKs (i.e. Src family PTKs, JAKs) are recruited to the cytoplasmic domain of the engaged receptor. Execution of these initial phosphorylations and their translation into efficient cellular stimulation requires concomitant activation of diverse signaling pathways. Availability of stable, preassembled matrices at the plasma membrane would facilitate scaffolding of a large array of receptors, coreceptors, tyrosine kinases and other signaling and adapter proteins, as it is the case in signaling via the T cell antigen receptor. The concept of the signaling platform has gained usage to characterize the membrane structure where many different membrane-bound components need to be assembled in a coordinated manner to carry out signaling. The structural basis of the signaling platform lies in preferential assembly of certain classes of lipids into distinct physical and functional compartments within the plasma membrane. These membrane microdomains or rafts (Figure 1) serve as privileged sites where receptors and proximal signaling molecules optimally interact. In this review, we shall discuss first how signaling platforms are assembled and how receptors and their signaling machinery could be functionally linked in such structures. The second part of our review will deal with selected examples of raft-based signaling pathways in T lymphocytes and NK cells to illustrate the ways in which rafts may facilitate signaling.     

Notch signaling: simplicity in design, versatility in function

Notch signaling is evolutionarily conserved and operates in many cell types and at various stages during development. Notch signaling must therefore be able to generate appropriate signaling outputs in a variety of cellular contexts. This need for versatility in Notch signaling is in apparent contrast to the simple molecular design of the core pathway. Here, we review recent studies in nematodes, Drosophila and vertebrate systems that begin to shed light on how versatility in Notch signaling output is generated, how signal strength is modulated, and how cross-talk between the Notch pathway and other intracellular signaling systems, such as the Wnt, hypoxia and BMP pathways, contributes to signaling diversity.     

Trask phosphorylation defines the reverse mode of a phosphotyrosine signaling switch that underlies cell anchorage state

Phosphotyrosine signaling in anchored epithelial cells constitutes a spacially ordained signaling program that largely functions to promote integrin-linked focal adhesion complexes, serving to secure cell anchorage to matrix and as a bidirectional signaling hub that coordinates the physical state of the cell and its environment with cellular functions including proliferation and survival. Cells release their adhesions during processes such as mitosis, migration or tumorigenesis, but the fate of signaling through tyrosine phosphorylation in unanchored cells remains poorly understood. In an examination of epithelial cells in the unanchored state, we find abundant phosphotyrosine signaling, largely recommitted to an anti-adhesive function mediated through the Src family phosphorylation of their transmembrane substrate Trask/CDCP1/gp140. Src-Trask phosphorylation inhibits integrin clustering and focal adhesion assembly and signaling, defining an active phosphotyrosine signaling program underlying the unanchored state. Src-Trask signaling and Src-focal adhesion signaling inactivate each other, constituting two opposing modes of phosphotyrosine signaling that define a switch underline cell anchorage state. Src kinases are prominent drivers of both signaling modes, identifying their position at the helm of adhesion signaling capable of specifying anchorage state through substrate selection. These experimental studies along with concurring phylogenetic evidence suggest that phosphorylation on tyrosine is a signaling function fundamentally linked with the regulation of integrins.Key words: Trask, CDCP1, gp140, tyrosine phosphorylation, integrin, Src     

Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1

Fringe O-fucose-beta1,3-N-acetylglucosaminyltransferases modulate Notch signaling by potentiating signaling induced by Delta-like ligands, while inhibiting signaling induced by Serrate/Jagged1 ligands. Based on binding studies, the differential effects of Drosophila fringe (DFng) on Notch signaling are thought to result from alterations in Notch glycosylation that enhance binding of Delta to Notch but reduce Serrate binding. Here, we report that expression of mammalian fringe proteins (Lunatic [LFng], Manic [MFng], or Radical [RFng] Fringe) increased Delta1 binding and activation of Notch1 signaling in 293T and NIH 3T3 cells. Although Jagged1-induced signaling was suppressed by LFng and MFng, RFng enhanced signaling induced by either Delta1 or Jagged1, underscoring the diversity of mammalian fringe glycosyltransferases in regulating signaling downstream of different ligand-receptor combinations. Interestingly, suppression of Jagged1-induced Notch1 signaling did not correlate with changes in Jagged1 binding as found for Delta1. Our data support the idea that fringe glycosylation increases Delta1 binding to potentiate signaling, but we propose that although fringe glycosylation does not reduce Jagged1 binding to Notch1, the resultant ligand-receptor interactions do not effectively promote Notch1 proteolysis required for activation of downstream signaling events.